from pptx import Presentation

# 读入已有ppt文件
ppt = Presentation('write_text.pptx')

# 第一页幻灯片
slide0 = ppt.slides[0]
# 获得第一页幻灯片所有的占位符
placeholder = slide0.shapes.placeholders
# 在第二个占位符对象中添加新段落
new_paragraph = placeholder[1].text_frame.add_paragraph()
# 追加新内容
new_paragraph.text = '追加的新内容'
ppt.save('write_text3.pptx')